Bearing pressure-resistant member and process for making the same

ABSTRACT

A bearing pressure-resistant member, including a matrix and a carbide dispersed in the matrix, the carbide having an average particle size of not more than 0.3 μm. A carbon (C) content in an outer surface of the bearing pressure-resistant member is in a range of 0.6 to 1.5 mass percent. A process for making the bearing pressure-resistant member including subjecting a workpiece containing C to either of gas carburizing and gas carbonitriding to enhance the C content in the outer surface of the workpiece to 0.6 to 1.5 mass percent, holding the workpiece at a first temperature not more than an Ac 1  transformation point under reduced pressure, heating the workpiece to a second temperature not less than the Ac 1  transformation point under reduced pressure, followed by holding the workpiece at the second temperature, and subjecting the workpiece to quenching.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an element useable as a powertransmission part such as gears and rolling-contact bearings, whichnecessitates a high surface fatigue strength. More specifically, thisinvention relates to a bearing pressure-resistant member suitable foruse under a relatively high bearing pressure in a relatively hightemperature range of about 100° C. to about 300° C., and relates to aprocess for making the bearing pressure-resistant member.

[0002] There have been proposed methods for enhancing bearing pressurestrength of the above-described power transmission part. Among themethods, there are hyper-eutectoid carburizing and high densitycarburizing. In these carburizing methods, M₃C carbide such as Fe₃Cwhich is not readily decomposed in the above-described temperaturerange, is precipitated so that hardness of the part and resistance totempering softening can be enhanced. European Patent ApplicationPublication No. 1070760 A2 (corresponding to Japanese Patent ApplicationFirst Publication No. 2001-98343) discloses a bearing pressure-resistantmember including M₂₃C₆ carbide particles which are precipitated in thematrix in a dispersed state. M₂₃C₆ carbide is formed by carbon (C) andmetal combined with the C, which includes chromium (Cr), molybdenum (Mo)and the like. The precipitation of M₂₃C₆ carbide is performed byisothermal heat treatment carried out between carburizing and quenching,in which a workpiece is held at a temperature of not more than an Ac₁transformation point. The bearing pressure-resistant member of thisrelated technology has excellent surface fatigue strength as comparedwith steels in which M₃C carbide is precipitated in the matrix. M₂₃C₆carbide is also disclosed in U.S. Pat. No. 6,342,109 (issued Jan. 29,2002, to Takemura et al).

[0003] Such carbides are precipitated in an outer surface of the elementby isothermal heat treatment after subjecting the element to carburizingto increase a C content in the outer surface. In the isothermal heattreatment, the element carburized is held at a temperature at which Cproduces a solid solution with the matrix. This precipitation tends tostart at a region where Cr forming carbides is locally present, or atgrain boundaries of austenite particles at which nucleus of theprecipitated carbide is produced. The precipitated carbide is grown in ahomogeneously dispersed state in the outer surface of the element by theisothermal heat treatment. Thereafter, the element is subjected toquenching to provide the finished product.

[0004] Further, a heat treatment under controlled atmosphere (gascarburizing) is generally used as the carburizing method, which utilizesdenatured gas made from propane (C₃H₈) gas. In addition, U.S. Pat. No.6,258,179 B1 describes vacuum carburizing in which hydrogen (H₂)generated upon gas carburizing hardly infiltrates into a material steel.

SUMMARY OF THE INVENTION

[0005] However, it is difficult to homogeneously disperse M₂₃C₆ carbidein the entire structure of the element or the outer surface layerthereof which has an increased C content by carburizing, only by theabove-described isothermal heat treatment. Therefore, the element willbe provided with a region in which M₂₃C₆ carbide is not precipitated, sothat the region will fail to be sufficiently hardened. Further, at thequenching step following the above-described holding step, dense texturewill not be obtained at the region having no carbide precipitated anddispersed, causing deterioration in rolling fatigue strength. This isbecause carbide can prevent martensite produced upon quenching fromcoarsely growing up. In European Patent Application Publication No.1070760 A2 as discussed above, a region having no M₂₃C₆ carbideprecipitated will be generated or an excessively long treatment timewill be required at the holding step. This causes increase in theproduction cost.

[0006] Further, residual H₂ infiltrating into the metal of the elementduring the gas carburizing as described above, can be remarkably reducedby the subsequent tempering. However, if the residual H₂ isinsufficiently reduced, the element will suffer from delayed fracture ordeterioration in bending fatigue strength and toughness. Recently, ithas been recognized that rolling fatigue lives of rolling elements whichundergo high bearing pressure upon coming into rolling contact withcounterparts, will be considerably deteriorated due to the residual H₂.Furthermore, since H₂ is readily absorbed into the above-describedcarbide as well known, the residual H₂ must be reduced in the gascarburizing treatment. In order to reduce the residual H₂, there hasbeen proposed baking in which an element is held at temperingtemperature or less for several ten hours. This leads to decrease inproduction efficiency and increase in production cost. On the contrary,if the holding temperature is raised beyond a certain level, the elementwill be softened while the treatment time can be reduced.

[0007] In U.S. Pat. No. 6,258,179 B1 as discussed above, coarse carbidetends to be produced at grain boundaries of austenite grains, so thatrolling fatigue strength or bending fatigue strength will bedeteriorated. In addition, apparatus for use in the vacuum carburizingtreatment is relatively expensive.

[0008] An object of the present invention is to provide a bearingpressure-resistant member for use in power transmission, which hasexcellent surface fatigue strength with provision of M₂₃C₆ carbidehomogeneously and finely dispersed in the matrix. Also, the object ofthe present invention is to provide a process for making the bearingpressure-resistant member by holding the element at a predeterminedtemperature to precipitate M₂₃C₆ carbide in the matrix in homogeneouslyand finely dispersed state, the process being capable of improvingsignificantly surface fatigue strength. A further object of the presentinvention is to provide a bearing pressure-resistant member for use inpower transmission, which is reduced in residual H₂ even by gascarburizing or gas carbonitriding, and enhanced bending fatigue strengthor rolling fatigue strength, and a process for making the bearingpressure-resistant member, capable of suppressing deterioration inbending fatigue strength or rolling fatigue strength which will becaused due to delayed fracture or hydrogen embrittlement.

[0009] According to one aspect of the present invention, there isprovided a bearing pressure-resistant member, comprising:

[0010] a matrix and a carbide dispersed in the matrix, said carbidehaving an average particle size of not more than 0.3 μm; and

[0011] a C content ranging from 0.6 to 1.5 mass percent in an outersurface thereof.

[0012] According to a further aspect of the present invention, there isprovided a process for making a bearing pressure-resistant member,comprising:

[0013] subjecting a workpiece containing C to either of gas carburizingand gas carbonitriding to enhance a C content in an outer surface of theworkpiece to 0.6 to 1.5 mass percent;

[0014] holding the workpiece at a first temperature not more than an Ac₁transformation point under reduced pressure;

[0015] heating the workpiece to a second temperature not less than theAc₁ transformation point under reduced pressure, followed by holding theworkpiece at the second temperature; and

[0016] subjecting the workpiece to quenching.

[0017] According to a still further aspect of the present invention,there is provided a process for making a bearing pressure-resistantmember, comprising:

[0018] heating a workpiece made of a mechanical structural steelcontaining C in an amount ranging from 0.6 to 1.5 mass percent, to afirst temperature of 600° C. to 750° C. wherein the workpiece is heatedat a rate of 0.2° C. to 30° C. per minute in a temperature range of 500°C. to 650° C.;

[0019] holding the workpiece at the first temperature;

[0020] heating the workpiece to a second temperature not less than anAc₁ transformation point and not more than an Acm transformation point,followed by holding the workpiece at the second temperature; and

[0021] subjecting the workpiece to quenching.

[0022] According to a still further aspect of the present invention,there is provided a process for making a bearing pressure-resistantmember, comprising:

[0023] heating a workpiece made of a mechanical structural steelcontaining C in an amount ranging from 0.6 to 1.5 mass percent, Cr in anamount ranging from 1.2 to 3.2 mass percent, and Mo in an amount rangingfrom 0.25 to 2.0 mass percent, to a first temperature of 600° C. to 750°C. under reduced pressure wherein the workpiece is heated at a rate of0.2° C. to 30° C. per minute in a temperature range of 500° C. to 650°C. under reduced pressure;

[0024] holding the workpiece at the first temperature;

[0025] heating the workpiece to a second temperature not less than anAc₁ transformation point and not more than a predetermined temperature Trepresented by the following formula:

T(° C.)=675+120.Si(%)−27.Ni(%)+30.Cr(%)+215.Mo(%)−400V(%);

[0026] holding the workpiece at the second temperature; and

[0027] subjecting the workpiece to quenching.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is an explanatory diagram illustrating first heat patternof heat treatment carried out in examples of the present invention;

[0029]FIG. 2 is an explanatory diagram illustrating second heat patternof heat treatment carried out in the examples of the present invention;

[0030]FIG. 3 is an explanatory diagram illustrating a condition of gascarburizing carried out in the examples of the present invention;

[0031]FIG. 4 is an explanatory diagram illustrating third heat patternof heat treatment carried out in the examples of the present inventionafter the gas carburizing of FIG. 3;

[0032]FIG. 5 is a scanning electron microphotograph of a cross-sectionof an outer surface of a specimen of Comparative Example 1;

[0033]FIG. 6 is an explanatory diagram illustrating fourth heat patternof heat treatment carried out in the examples of the present invention;

[0034]FIGS. 7A and 7B are respectively a plan view and a side view ofspecimens used in the examples of the present invention; and

[0035]FIGS. 8A and 8B show a schematic diagram of a thrust rollingfatigue tester used in examples of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0036] A bearing pressure-resistant member of the present inventionincludes a matrix and a carbide dispersed in the matrix. The carbide hasan average particle size of not more than 0.3 μm. The bearingpressure-resistant member has a carbon (C) content ranging from 0.6 to1.5 mass percent in at least an outer surface thereof. The carbide isprecipitated in the matrix in uniformly and finely dispersed state.Owing to the above-described carbide, the bearing pressure-resistantmember of the invention can be improved in bearing fatigue strength. Thebearing pressure-resistant member of the invention can be suitably usedas parts such as gears and rolling-contact bearings which are used underhigh bearing pressure.

[0037] The bearing pressure-resistant member may be made of a mechanicalstructural steel containing C in an amount ranging from 0.6 to 1.5 masspercent. More preferably, the bearing pressure-resistant member may bemade of a mechanical structural steel containing C in an amount rangingfrom 0.6 to 1.5 mass percent, Cr in an amount ranging from 1.2 to 3.2mass percent, and Mo in an amount ranging from 0.25 to 2.0 mass percent.The carbide is preferably M₂₃C₆ carbide containing at least Cr. Further,more preferably, the bearing pressure-resistant member may be made of amechanical structural steel which contains Cr in an amount ranging from1.2 to 3.2 mass percent and Mo in an amount ranging from 0.25 to 2.0mass percent and is treated by either of carburizing and carbonitridingsuch that the C content is in the range of 0.6 to 1.5 mass percent.

[0038] Specifically, if the C content is below 0.6 mass percent, anamount of the carbide which is required for obtaining appropriatehardness cannot be precipitated. The carbide amount can be indicated asa ratio of an area of the carbide to a reference area of the bearingpressure-resistant member. If the C content is more than 1.5 masspercent, network of M₃C carbide will be produced to thereby deterioratemechanical properties of the bearing pressure-resistant member. Further,if the average particle size of the carbide is more than 0.3 μm, rollingfatigue life of the bearing pressure-resistant member will be reduced.Furthermore, if the Cr content in the material steel is less than 1.2mass percent, the amount of carbide will be decreased so that thebearing pressure-resistant member having excellent rolling fatigue lifecannot be obtained. On the contrary, if the Cr content is more than 3.2mass percent, machinability of the bearing pressure-resistant memberwill be deteriorated. Further, if the Mo content in the material steelis less than 0.25 mass percent, the M₂₃C₆ carbide will not be stablyprecipitated. If the Mo content in the material steel is less than 2.0mass percent, the machinability of the bearing pressure-resistant memberwill be decreased. Owing to the Cr content and the Mo content which arein the above-described ranges, respectively, precipitation of the M₂₃C₆carbide can be facilitated. This serves for assuring good rollingfatigue strength of the bearing pressure-resistant member even at therelatively high temperature range of about 100° C. to about 300° C.Further, the M₂₃C₆ carbide is finely dispersible in the matrix ascompared to other carbides. Therefore, the bearing pressure-resistantmember containing the M₂₃C₆ carbide precipitated at the finely dispersedstate in the matrix, can be significantly improved in rolling fatiguestrength.

[0039] The C content in the outer surface of the bearingpressure-resistant member may be controlled to 0.6 to 1.5 mass percentby either of carburizing and carbonitriding. Namely, the amount of Ccontained in the outer surface of the bearing pressure-resistant membermay be enhanced to 0.6 to 1.5 mass percent by either of carburizing andcarbonitriding. Alternatively, the amount of C contained in the outersurface of the bearing pressure-resistant member may be enhanced to 0.6to 1.5 mass percent in by either of gas carburizing and gascarbonitriding. Gas carburizing and gas carbonitriding can be conductedwith a cost-saving facility and at well-controlled carburizingconcentration. This contributes to reduction of the production cost ofthe bearing pressure-resistant member and to suppression of coarsegrowth of carbide in the matrix. In addition, the bearingpressure-resistant member of the invention can exhibit high hardnessand, therefore, excellent bearing fatigue strength. Further, upon thegas carburizing or gas carbonitriding, a whole amount of hydrogen (H₂)released over a temperature range of 100° C. to 900° C., may be limitedto not more than 0.2 ppm. This can prevent hydrogen embrittlement inhigh-hardness material steel which will occur due to adsorption of H₂ tothe M₂₃C₆ carbide. The bearing pressure-resistant member of theinvention, therefore, can exhibit increased bearing fatigue strength andexcellent rolling fatigue life.

[0040] A process for making the bearing pressure-resistant member,according to the invention, is now explained. A workpiece containing Cis subjected to either of gas carburizing and gas carbonitriding toenhance the amount of C contained in an outer surface of the workpieceto 0.6 to 1.5 mass percent. Next, the workpiece is subjected to a firstisothermal heat treatment under reduced pressure. In the firstisothermal heat treatment, the workpiece is held at a first temperaturenot more than an Ac₁ transformation point for a first period of time.Subsequently, the workpiece is subjected to a second isothermal heattreatment under reduced pressure. In the second isothermal heattreatment, the workpiece is heated to a second temperature not less thanthe Ac₁ transformation point, and then held at the second temperaturefor a second period of time. Subsequent to the holding at the secondtemperature, the workpiece is subjected to quenching to provide thebearing pressure-resistant member.

[0041] In the first isothermal heat treatment, an amount of H₂ containedin the workpiece is effectively reduced. In the second isothermal heattreatment and the quenching subsequent thereto, a tough structure ofmartensite or bainite is produced. The first isothermal heat treatmentis conducted in advance of the second isothermal heat treatment. By thefirst isothermal heat treatment, the H₂ content in the workpiece iseffectively reduced without deteriorating the hardness of the workpiecein spite of the first temperature higher than the temperature of theconventional baking. Further, since the first temperature is not morethan the Ac₁ transformation point, there is shown no increase in grainsize of austenite and no coarse growth of carbide present at grainboundaries of austenite grains. In addition, the process of theinvention which employs gas carburizing or gas carbonitriding, can servefor saving the facility cost and readily controlling the carburizingconcentration as explained above. According to the process of theinvention, the bearing pressure-resistant member having excellentbearing fatigue strength and rolling fatigue life can be produced withan inexpensive production cost.

[0042] Specifically, the workpiece may be made of a mechanicalstructural steel containing Cr in an amount ranging from 1.2 to 3.2 masspercent and Mo in an amount ranging from 0.25 to 2.0 mass percent. Thefirst temperature may be in a range of 600° C. to 750° C. which isreached by heating the workpiece at a rate of 0.2° C. to 30° C. perminute in a temperature range of 500° C. to 650° C. The secondtemperature may be in the range not less than the Ac₁ transformationpoint but not more than a predetermined temperature T represented by thefollowing formula:

T(° C.)=675+120.Si(%)−27.Ni(%)+30.Cr(%)+215.Mo(%)−400V(%).

[0043] In the first isothermal heat treatment, the H₂ content in theworkpiece is reduced, and at the same time, M₂₃C₆ carbide ishomogeneously precipitated in the matrix. Upon heating the workpiece tothe first temperature, the temperature rise rate is 0.2° C. to 30° C.per minute in the range of 500° C. to 650° C. wherein nucleus of theM₂₃C₆ carbide is produced. This facilitates producing the nucleus of theM₂₃C₆ carbide, so that the M₂₃C₆ carbide can be precipitated at morefinely and homogeneously dispersed state in the matrix. By the secondisothermal heat treatment subsequent to the first isothermal heattreatment, and the quenching, the precipitated M₂₃C₆ carbide can be keptin the homogeneously and finely dispersed state without formation ofexcessive solid solution. Then, the workpiece has martensite or bainitestructure in the matrix. As a result, the bearing pressure-resistantmember of the invention can be significantly increased in rollingfatigue strength.

[0044] A second process for making the bearing pressure-resistantmember, according to the present invention, is explained. A workpiecemade of a mechanical structural steel containing C in an amount rangingfrom 0.6 to 1.5 mass percent is heated to a first temperature of 600° C.to 750° C. wherein the workpiece is heated at a rate of 0.2° C. to 30°C. per minute in a temperature range of 500° C. to 650° C. The workpieceis then held at the first temperature for a first period of time. Next,the workpiece is heated to a second temperature not less than an Ac₁transformation point but not more than an Acm transformation point, andthen held at the second temperature for a second period of time.Subsequent to the holding at the second temperature, the workpiece issubjected to quenching.

[0045] The workpiece may be subjected to either of carburizing andcarbonitriding to enhance a C content in an outer surface of theworkpiece to 0.6 to 1.5 mass percent before heating the workpiece to thefirst temperature. Specifically, in a case where the C content in theouter surface of the bearing pressure-resistant member is less than 0.6mass percent, the amount of carbide required for appropriate hardnesscannot be precipitated. In other words, a required ratio of an area ofcarbide precipitated in a local region of the outer surface of thebearing pressure-resistant member, to the whole area of the local regionthereof cannot be obtained. If the C content in the outer surface of thebearing pressure-resistant member exceeds 1.5 mass percent, M₃C carbidewill be produced to thereby deteriorate mechanical properties of thebearing pressure-resistant member.

[0046] Further, if the first temperature is below 600° C., a rate ofdiffusion of C will decrease, whereby the growth of M₂₃C₆ carbide willbe significantly lowered. This causes increase in the production cost.If the first temperature is more than 750° C., the C content in thematrix will be consumed when the M₃C carbide is produced. M₂₃C₆ carbide,therefore, cannot grow up so that the hardness of the bearingpressure-resistant member will not be assured. Furthermore, if thetemperature rise rate is less than 0.2° C. per minute in the range of500° C. to 650° C., the treatment time is excessively prolonged, causingincrease in the production cost. If the temperature rise rate is morethan 30° C. per minute in the range of 500° C. to 650° C., a timerequired for producing the nucleus of M₂₃C₆ carbide will be too short.This will cause lack of the amount of carbide precipitated. Preferably,the temperature rise rate is in a range of 0.2° C. to 5° C. per minutein the range of 500° C. to 650° C.

[0047] Further, if the second temperature is less than the Ac₁transformation point, the matrix obtained after quenching will not havemartensite or bainite structure. If the second temperature is more thanthe Acm transformation point or temperature T as given by theabove-described formula, the M₂₃C₆ carbide precipitated will form thesolid solution again.

[0048] A third process for making the bearing pressure-resistant member,according to the present invention, is explained. A workpiece made of amechanical structural steel containing C in an amount ranging from 0.6to 1.5 mass percent, Cr in an amount ranging from 1.2 to 3.2 masspercent, and Mo in an amount ranging from 0.25 to 2.0 mass percent, isheated to a first temperature of 600° C. to 750° C. under reducedpressure. In the heat treatment, the workpiece is heated at a rate of0.2° C. to 30° C. per minute in a temperature range of 500° C. to 650°C., and held at the first temperature for a first period of time. Next,the workpiece is heated to a second temperature not less than an Ac₁transformation point and not more than a predetermined temperature Trepresented by the following formula:

T(° C.)=675+120.Si(%)−27.Ni(%)+30.Cr(%)+215.Mo(%)−400V(%)

[0049] The workpiece is then held at the second temperature for a secondperiod of time. Subsequently, the workpiece is subjected to quenching.

[0050] The workpiece may be subjected to either of carburizing andcarbonitriding to enhance a C content in an outer surface of theworkpiece to 0.6 to 1.5 mass percent before heating the workpiece to thefirst temperature of 600° C. to 750° C.

[0051] The reasons for limitation of the temperature ranges andcomponent values as described above are summarized as follows. Here, thecontents of the respective components are represented by mass percent.

[0052] Outer surface C content: 0.6% to 1.5% If the C content is lessthan 0.6%, appropriate hardness of the outer surface of the bearingpressure-resistant member cannot be assured. If the C content is morethan 1.5%, M₃C carbide will be coarsely precipitated at the grainboundary of austenite grains. In this case, good bearing fatiguestrength cannot be obtained.

[0053] Whole amount of H₂:0.2 ppm or less

[0054] If a whole amount of H₂ is more than 0.2 ppm, hydrogenembrittlement will occur, causing deterioration of rolling fatigue lifeof a steel having a high C content and high hardness.

[0055] First isothermal heat treatment (H₂ reduction treatment)subsequent to either of gas carburizing, gas carbonitriding, carburizingand carbonitriding: temperature not more than Ac₁ transformation pointunder reduced pressure

[0056] If the temperature is more than the Ac₁ transformation point,undesired coarse carbide will be precipitated at grain boundary ofaustenite grains. The reduced pressure facilitates reduction of H₂infiltrated into the steel and prevents occurrence of decarburizationtherein.

[0057] Quenching temperature: Ac₁ transformation point or more

[0058] If the temperature is less than the Ac₁ transformation point, asteel will not have martensite or bainite structure in the matrix.Therefore, required hardness of the bearing pressure-resistant membercannot be obtained.

[0059] Cr content: 1.2% to 3.2%

[0060] Since Cr is an essential for producing M₂₃C₆ carbide, it ispreferable that a material steel contains Cr in an amount of 1.2% to3.2%. If the Cr content is less than 1.2%, an amount of the M₂₃C₆carbide precipitated will be decreased or stable precipitation of theM₂₃C₆ carbide cannot be attained. If the Cr content is more than 3.2%,production cost of the bearing pressure-resistant member will beincreased and machinability thereof will be deteriorated.

[0061] Mo content: 0.25% to 2.0%

[0062] Mo is an element for producing M₂₃C₆ carbide. It is preferablethat a material steel contains Mo in an amount of 0.25% to 2.0%. If theMo content is less than 0.25%, an amount of the M₂₃C₆ carbideprecipitated will be decreased or stable precipitation of the M₂₃C₆carbide cannot be attained. If the Mo content exceeds 2.0%, there willoccur increase in production cost of the bearing pressure-resistantmember and deterioration of machinability thereof.

[0063] Average particle size of M₂₃C₆ carbide: 0.3 μm or less

[0064] If the average particle size of M₂₃C₆ carbide is more than 0.3μm, too long time will be required for obtaining a uniform structure ofthe bearing pressure-resistant member in which the precipitated M₂₃C₆carbide is finely and uniformly dispersed in the matrix. This will causeincrease in production cost. If the average particle size of M₂₃C₆carbide is excessively larger than 0.3 μm, rolling fatigue life of thebearing pressure-resistant member will be deteriorated.

[0065] Temperature of first isothermal heat treatment subsequent toeither of gas carburizing, gas carbonitriding, carburizing andcarbonitriding: 600° C. to 750° C.

[0066] If the temperature is less than 600° C., a rate of diffusion of Cinto the matrix will be small so that the growth of M₂₃C₆ carbide willbe considerably slow. This will cause increase in production cost of thebearing pressure-resistant member. If the temperature is more than 750°C., C will be consumed for production of M₃C carbide so that the growthof M₂₃C₆ carbide cannot be caused. As a result, required hardness of thebearing pressure-resistant member cannot be obtained.

[0067] Temperature rise rate upon heating up to the isothermal heattreatment temperature of 600° C. to 750° C.: 0.2° C. to 30° C. perminute

[0068] If the temperature rise rate is less than 0.2° C./min, thetreatment time will be remarkably prolonged, causing increase inproduction cost of the bearing pressure-resistant member. If thetemperature rise rate is more than 30° C./min, nucleus of M₂₃C₆ carbidewill not be sufficiently formed so that the M₂₃C₆ carbide will not beuniformly and finely precipitated in the matrix.

[0069] Quenching temperature: not less than Ac₁ transformation point andnot more than temperature T represented by the following formula: T(°C.)=675+120.Si(%)−27.Ni(%)+30.Cr(%)+215.Mo(%)−400V(%) If the quenchingtemperature is less than the Ac₁ transformation point, the matrix of thebearing pressure-resistant member will not have martensite or bainitestructure. If the quenching temperature is more than the above-describedtemperature T, M₂₃C₆ carbide precipitated in the matrix will form thesolid solution again.

[0070] The production process of the invention can provide a bearingpressure-resistant member having fine M₂₃C₆ carbide at least the outersurface, which is precipitated in the matrix in the homogeneouslydispersed state as explained above. Further, in the process of theinvention, reduction of H₂ can be effectively performed to therebyprevent hydrogen embrittlement which will be caused by hydrogeninfiltration into the matrix. Accordingly, the bearingpressure-resistant member made by the process of the invention canexhibit excellent bearing fatigue strength and enhanced rolling fatiguelife because of the M₂₃C₆ carbide finely and homogeneously dispersed.

EXAMPLES

[0071] The present invention is described in more detail by way ofexamples and comparative examples by referring to the accompanyingdrawings. However, these examples are only illustrative and not intendedto limit a scope of the present invention thereto.

Examples 1-6 and Comparative Examples 1-6

[0072] Specimens were prepared from six steel materials A-F for machinestructural use, each having a chemical composition as shown in Table 1,in the following manner. The specimens made of materials A, B and C weresubjected to carburizing so as to have a C content of 0.7 to 1.4 masspercent in an outer surface thereof. Next, the specimens were subjectedto heat treatment including first and second isothermal heat treatmentsand then quenching. In the heat treatment, either of heat patterns shownin FIGS. 1 and 2 was used. In FIGS. 1 and 2, t1 represents a firsttemperature for the first isothermal heat treatment, and t2 represents asecond temperature for the second isothermal heat treatment. Also, dTrepresents a temperature rise rate per minute in a temperature range of500° C. to 650° C. Subsequently, the specimens heat-treated weresubjected to tempering at a temperature of 170° C. for two hours, andthen to grinding to finish the outer surface. The specimens made ofmaterials D, E and F were not subjected to carburizing but subjected tothe same heat treatment as described above. The specimens heat-treatedwere then subjected to tempering and grinding in the same manner asdescribed above. Meanwhile, in Example 3, the specimen was cooled downto a temperature of 600° C. after carburizing, and then was subjected tothe same heat treatment as described above. TABLE 1 Steel ChemicalComposition (mass %) T* Material C Si Mn Ni Cr Mo V (° C.) A 0.18 1.411.11 1.48 3.15 0.44 0.33  860 B 0.20 1.03 0.30 2.10 2.18 1.21 — 1067 C0.33 0.82 0.24 2.32 1.62 1.88 0.13 1112 D 0.65 1.55 0.34 1.52 2.65 0.990.21 1028 E 0.87 0.82 0.18 2.00 2.02 0.78 —  948 F 1.18 1.10 0.28 1.041.81 1.44 0.18 1071

[0073] Each of the thus-prepared specimens was cut into a disk shapeshown in FIGS. 7A and 7B, which had a diameter of 60 mm and a thicknessof 5 mm. The cut specimen was subjected to rolling fatigue test underconditions shown in Table 2, to evaluate the rolling fatigue life, i.e.,the life up to flaking. The rolling fatigue life was obtained as L50 ata cumulative fracture probability of 50% based on Weibull distribution.The rolling fatigue test was conduced using a thrust rolling fatiguetester shown in FIGS. 8A and 8B. As illustrated in FIG. 8B, specimen 3was set in contact with three steel bearing balls 5 in traction 5 oil 4.In FIG. 8B, only two steel bearing balls 5 were shown. Specimen 3 wasbrought into rolling contact with steel bearing balls 5 when a shaft ofthe tester rotates about the rotation axis as indicated by curved arrowof FIG. 8B. The results were shown in Table 3. TABLE 2 Testing MachineThrust rolling fatigue tester Bearing Pressure 5.23 GPa Maximum ShearingStress Depth* 0.1 mm from outer-most surface Revolution Number 2000 rpmLubricating Oil Transmission oil Oil Temperature 150° C. CounterpartSteel Ball Three balls made of JIS SUJ2 steel, having ⅜-inch diameter

[0074] Further, each of the thus-prepared specimens was subjected toevaluation of properties of the outer surface in the following manner.The specimen was cut vertically, and the vertical cross section wassubjected to etching with a nital etchant composed of 3% nitric acidalcohol solution. Microphotograph at the magnification of 10,000 of anouter portion of the vertical cross section was obtained using ascanning electron microscope (SEM). The outer portion had a depth of 0.1mm from the outer-most surface of the specimen. The microphotograph wassubjected to image analysis to measure an average particle size ofprecipitated carbide. Further, a local region of the vertical crosssection was observed with an optical microscope to obtain an area ratioof non-carbide-precipitated portion where no carbide was precipitated,to the whole local region. The local region was located at a depth of0.1 mm +/− 0.05 mm from the outer-most surface of the specimen. Theresults were shown in Table 3. TABLE 3 Steel Heat Heat TreatmentMaterial Pattern dT (° C./min) t1 (° C.) t2 (° C.) Example 1 A 5 630 850Example 2 B 5 650 870 Example 3 C 10 690 880 Example 4 D 0.3 610 830Example 5 E 10 640 900 Example 6 F 15 730 850 Comparative A 5 630 880Example 1 Comparative B 5 770 870 Example 2 Comparative C 10 580 880Example 3 Comparative D 40 610 830 Example 4 Comparative E 10 640 950Example 5 Comparative F 15 760 850 Example 6 Area Ratio of AverageParticle Non-Carbide- Rolling Fatigue Size of Precipitated Life (L50) ×M₂₃C₆ Carbide (μm) Region (%) 10000 Example 1 0.18 0 880 Example 2 0.170 1000 Example 3 0.22 0 1000 Example 4 0.16 0 960 Example 5 0.21 0 1000Example 6 0.27 0 1000 Comparative 0.12 33 300 Example 1 Comparative 0.3118 290 Example 2 Comparative 0.21 45 320 Example 3 Comparative 0.09 21210 Example 4 Comparative 0.14 62 180 Example 5 Comparative 0.29 14 260Example 6

[0075] It was recognized from Table 3 that carbides 5 were precipitatedin the outer surfaces of the specimens obtained in Examples 1-6. Theprecipitated carbides were finely and homogeneously dispersed in thematrix so that the specimens obtained in Examples 1-6 exhibited theexcellent rolling fatigue lives. On the contrary, it was found that thespecimens obtained in Comparative Examples 1 and 5 had the regions ofthe outer surfaces which had no precipitated carbide because theprecipitated carbide formed a solid solution with the matrix again dueto the relatively high temperature t2. It was found that the specimensobtained in Comparative Examples 2 and 6 had too small amount of M₂₃C₆carbide precipitated in the outer surfaces due to the relatively hightemperature t1. It was found that the specimen obtained in ComparativeExample 3 had the region of the outer surface which had no precipitatedcarbide because the growth rate of carbide was small due to therelatively low temperature t1. Further, it was confirmed that thespecimens obtained in Comparative Example 4 had inhomogeneouslyprecipitated carbide in the outer surface which was caused by lessproduction of carbide nucleus due to the large temperature rise rate dT.As a result, the specimens obtained in Comparative Examples 1-6exhibited the shortened rolling fatigue lives as shown in Table 3.

Examples 7-10 and Comparative Examples 7-16

[0076] Specimens were prepared from five steel materials G-K for machinestructural use, each having a chemical composition as shown in Table 4,in the following manner. The specimens were subjected to normalizing andthen gas carburizing under conditions shown in FIG. 3. A carburizing gascomposition (C potential) was adjusted so as to provide a C content inan outer surface of each specimen in a range of 0.7 to 0.8 mass percent.Next, the specimens were subjected to heat treatment including first andsecond isothermal treatments and then quenching. In the heat treatment,a heat pattern shown in FIG. 4 was used under temperature conditions TC1to TC5 shown in Table 5. In FIG. 4, t1 and t2 and dT represent the sametemperatures and temperature rise rate as described in Examples 1-6 andComparative Examples 1-6. TABLE 4 Steel Chemical Composition (mass %) T*Material C Si Mn Ni Cr Mo V (° C.) G 0.20 0.93 0.26 2.32 1.50 0.96 0.19899 H 0.18 1.03 0.30 2.10 2.18 1.21 — 1067 I 0.21 0.25 0.72 — 0.93 — —733 J 0.21 0.22 0.69 — 1.06 0.21 — 778 K 0.19 0.18 0.56 1.79 0.57 0.28 —726

[0077] TABLE 5 Temperature Condition dT (° C./min) t1 (° C.) t2 (° C.)TC1 0.8 610 820 TC2 5 700 880 TC3 5 540 820 TC4 10 760 880 TC5 40 640820

[0078] Each of the thus-prepared specimens was subjected to evaluationof properties of the outer surface in the same manner as described inExamples 1-6 and Comparative Examples 1-6. The results were shown inTable 7.

[0079] On the other hand, each of the thus-prepared specimens was cutinto the same disk shape as described in Examples 1-6 and ComparativeExamples 1-6. The cut specimen was subjected to grinding to finish theouter surface. Thereafter, the specimen was subjected to the rollingfatigue test to evaluate the rolling fatigue life L10 at a cumulativefracture probability of 10% based on Weibull distribution. The rollingfatigue test was conducted using the same testing machine as describedin Examples 1-6 and Comparative Examples 1-6 except for using the testconditions shown in Table 6. The results were shown in Table 7. TABLE 6Testing Machine Thrust rolling fatigue tester Bearing Pressure (Load)5.2 GPa (10 kg) Revolution Number 2000 rpm Lubricating Oil NissanTraction Fluid KTF-1 Oil Temperature 150° C. Counterpart Steel BallThree balls made of JIS SUJ2 steel, having ⅜-inch diameter

[0080] TABLE 7 Area Ratio Average Rolling of Non- Particle FatigueTemper- Carbide- Size of Life Steel ature Precipitated Carbide (L10) ×Material Condition Region (%) (μm) 10000 Example 7 G TC1  0 0.19 79Example 8 G TC2  0 0.21 118 Example 9 H TC1  0 0.21 75 Example 10 H TC2 0 0.23 101 Comparative G TC3 38 0.17 11 Example 7 Comparative G TC5 310.23 14 Example 8 Comparative H TC3 27 0.20 31 Example 9 Comparative HTC4 10 0.08 18 Example 10 Comparative I TC4 —* —* 8 Example 11Comparative I TC5 16 0.21 15 Example 12 Comparative J TC3 41 0.22 9Example 13 Comparative J TC4  5 0.12 19 Example 14 Comparative K TC3 840.13 17 Example 15 Comparative K TC4 —* —* 12 Example 16

[0081] It was confirmed from Table 7 that carbides were precipitated inthe outer surface of the specimens obtained in Examples 7-10. Theprecipitated carbides were finely and uniformly dispersed in the matrixso that the specimens obtained in Examples 7-10 exhibited the excellentrolling fatigue lives. On the other hand, it was found that thespecimens obtained in Comparative Examples 7, 9, 13 and 15 had theregions of the outer surface which had no precipitated carbide becausecarbide was grown at a small rate due to the remarkably low temperaturet1. Therefore, the specimens obtained in Comparative Examples 7, 9, 13and 15 exhibited the rolling fatigue lives less than those in Examples7-10. It was found that the specimens obtained in Comparative Examples10, 11, 14 and 16 had substantially no precipitated carbide in the outersurface due to the excessively high temperatures t1 and t2, andtherefore had the shortened rolling fatigue lives. Further, it was foundthat the specimens obtained in Comparative Examples 8 and 12 had theregions of the outer surface which had no precipitated carbide becauseless production of carbide nucleus was caused due to the largertemperature rise rate dT. As a result, the specimens obtained inComparative Examples 8 and 12 exhibited the shortened rolling fatiguelives as shown in Table 7.

[0082]FIG. 5 shows a microphotograph of the cross-section, taken withSEM, of the outer surface of the specimen obtained in ComparativeExample 1. In FIG. 5, a black part depicts the region havingprecipitated carbide, and a blank part depicts the region having noprecipitated carbide.

Examples 11-16 and Comparative Examples 17-25

[0083] Specimens were prepared in the following manner. The specimenswere made of four steel materials L-O for machine structural use, eachhaving a chemical composition as shown in Table 8. The specimens weresubjected to normalizing and then gas carburizing under conditions shownin FIG. 3. A carburizing gas composition (C potential) was adjusted soas to have such a C content in an outer surface of each specimen asshown in Table 10. Subsequently, the specimens were subjected to heattreatment in which a heat pattern shown in FIG. 6 was used undertemperature conditions TC6 to TC10 shown in Table 9, under reducedambient pressure of 100 Pa. In FIG. 6, t1, t2 and dT represent the sametemperatures and temperature rise rate as described in Examples 1-6 andComparative Examples 1-6. In Examples 11-16 and Comparative Examples 17,18 and 23-25, the specimens were subjected to the same tempering asdescribed in Examples 7-10 and Comparative Examples 7-16 after the heattreatment, but not held at temperature t3 for five hours. In ComparativeExamples 19-22, the specimens were not subjected to the first isothermalheat treatment at temperature t1, but subjected to the second isothermalheat treatment at temperature t2 and the same tempering and then held attemperature t3 for five hours. TABLE 8 Steel Chemical Composition (mass%) T* Material C Si Mn Ni Cr Mo V (° C.) L 0.20 1.00 0.30 2.00 1.50 1.500.19 1032.5 M 0.18 1.03 0.39 2.10 2.10 0.70 — 955 N 0.22 0.20 0.65 —1.50 0.20 — 787 O 0.20 1.00 0.30 1.90 — 0.70 — 894

[0084] TABLE 9 Temperature Condition dT (° C./min) t1 (° C.) t2 (° C.)t3 (° C.) TC6 5 650 820 — TC7 5 800 820 — TC8 35 650 880 — TC9 — — 880300  TC10 — — 820 120

[0085] Each of the thus-prepared specimens was cut into test pieceswhich were subjected to evaluation of properties. Hardness of each testpiece was measured using a Vickers hardness tester. The C content in thetest piece was measured by emission spectrochemical analysis. Themicrostructure of the precipitated carbide in the test piece wasidentified based on an electron beam diffraction image obtained byreplica method. Average particle size and area ratio of the precipitatedcarbide were obtained by image analysis of a SEM microphotograph of thetest piece. Whole amount of H₂ was obtained from a characteristic curveshowing a H₂ amount released upon heating the specimens from 100° C. to900° C. The results were shown in Table 7.

[0086] On the other hand, each of the thus-prepared specimens was cutinto the same disk shape as described in Examples 1-6 and ComparativeExamples 1-6. The specimen was subjected to grinding to finish the outersurface. Next, the specimen was subjected to the rolling fatigue test toevaluate the same rolling fatigue life L50 as described in Examples 1-6and Comparative Examples 1-6. The rolling fatigue test was conductedusing the same testing machine under the same test conditions asdescribed in Examples 7-10 and Comparative Examples 7-16. In ComparativeExamples 23-25, H₂ was charged into the rolling fatigue tester toexamine the influence of H₂ on the rolling fatigue lives of thespecimens. The results were shown in Table 10. TABLE 10 Surface Temper-Surface Steel C Content ature Hardness H₂ Amount Material (mass %)Condition (Hv) (ppm) Example 11 L 1.23 TC6 803 — Example 12 L 0.91 TC6762 — Example 13 M 1.32 TC6 794 — Example 14 M 0.82 TC6 755 — Example 15L 0.92 TC7 766 — Example 16 L 0.88 TC8 754 — Comparative L 1.65 TC6 798— Example 17 Comparative L 0.51 TC6 470 — Example 18 Comparative L 0.89TC9 620 0.4 Example 19 Comparative L 0.89  TC10 768 0.7 Example 20Comparative N 0.96  TC10 753 0.7 Example 21 Comparative O 0.82  TC10 7410.6 Example 22 Comparative L 1.23 TC6 803 0.7 Example 23 Comparative L1.23 TC6 803 1.2 Example 24 Comparative L 1.23 TC6 803 1.9 Example 25Precipitated Carbide Average Micro- Particle Area Rolling Fatiguestructure size (μm) Ratio (%) Life (L50) × 10⁷ Example 11 M₂₃C₆ 0.22 1311.5 Example 12 M₂₃C₆ 0.17  9 10.2 Example 13 M₂₃C₆ 0.20 16 10.9 Example14 M₂₃C₆ 0.19  8 9.8 Example 15 M₃C 7.25  1 2.1 Example 16 M₂₃C₆ 0.35  45.1 Comparative M₃C + M₂₃C₆ 3.46 22 0.1 Example 17 Comparative —* —* —*0.4 Example 18 Comparative —* —* —* 0.9 Example 19 Comparative —* 0.19 1 1.8 Example 20 Comparative M₃C 0.15  1 0.4 Example 21 Comparative —*—* —* 1.1 Example 22 Comparative M₂₃C₆ 0.22 13 1.3 Example 23Comparative M₂₃C₆ 0.22 13 0.4 Example 24 Comparative M₂₃C₆ 0.22 13 0.3Example 25

[0087] It was found from Table 10 that no H₂ released from the specimensobtained in Examples 11-16 was detected and the specimens exhibited theexcellent rolling fatigue lives. On the contrary, it was found that thespecimen obtained in Comparative Example 17 exhibited the deterioratedrolling fatigue life because network of M₃C carbide was precipitated atgrain boundaries of austenite particles due to too large C content inthe outer surface of the specimen. It was found that the specimenobtained in Comparative Example 18 exhibited the deteriorated rollingfatigue life because sufficient hardness was not obtained due to toosmall C content in the outer surface of the specimen. It was found thatthe specimen obtained in Example 15 had no M₂₃C₆ carbide precipitatedbut coarse M₃C carbide precipitated at grain boundaries of austeniteparticles because the first temperature t1 was more than Ac₁transformation point. As a result, the specimen obtained in Example 15exhibited the rolling fatigue life shorter than that of the specimenobtained in each of Examples 11-14. The specimen obtained in Example 15is suitably applicable to low-power automobiles. It was found that thespecimen obtained in Example 16 exhibited non-uniformity inprecipitation of carbide because production of nucleus of M₂₃C₆ carbidewas insufficient due to the large temperature rise rate dT. The specimenobtained in Example 16, therefore, had the shorter rolling fatigue lifethan that of the specimen obtained in each of Examples 11-14. Thespecimen obtained in Example 16 also is suitably applicable to low-powerautomobiles. Further, it was found that the specimen obtained inComparative Example 19 had no M₂₃C₆ carbide precipitated in the outersurface and the lower surface hardness because the first isothermal heattreatment was omitted. The specimen obtained in Comparative Example 19,therefore, exhibited the deteriorated rolling fatigue life. It was foundthat the specimen obtained in each of Comparative Examples 20-22 had noM₂₃C₆ carbide precipitated in the outer surface and the H₂ amountinsufficiently reduced, because the first isothermal heat treatment wasomitted and the temperature t3 after tempering at 170° C. was lower. Asa result, the specimens obtained in Comparative Examples 20-22 exhibitedthe deteriorated rolling fatigue lives. Further, it was confirmed thatthe specimens obtained in Comparative Examples 23-25 exhibited therolling fatigue lives which were deteriorated as the residual H₂ amountincreased.

[0088] This application is based on prior Japanese Patent ApplicationsNo. 2001-148517 filed on May 17, 2001 and No. 2001-160694 filed on May29, 2001, the entire contents of which are hereby incorporated byreference.

[0089] Although the invention has been described above by reference tocertain examples of the invention, the invention is not limited to theexamples described above. Modifications and variations of the examplesdescribed above will occur to those skilled in the art in light of theabove teachings. The scope of the invention is defined with reference tothe following claims.

What is claimed is:
 1. A bearing pressure-resistant member, comprising:a matrix and a carbide dispersed in the matrix, said carbide having anaverage particle size of not more than 0.3 μm; and a C content rangingfrom 0.6 to 1.5 mass percent in an outer surface thereof.
 2. The bearingpressure-resistant member as claimed in claim 1, wherein said bearingpressure-resistant member is made of a mechanical structural steelcontaining C in an amount ranging from 0.6 to 1.5 mass percent.
 3. Thebearing pressure-resistant member as claimed in claim 1, wherein saidbearing pressure-resistant member is made of a mechanical structuralsteel containing C in an amount ranging from 0.6 to 1.5 mass percent, Crin an amount ranging from 1.2 to 3.2 mass percent, and Mo in an amountranging from 0.25 to 2.0 mass percent, said carbide comprising M₂₃C₆carbide containing at least Cr.
 4. The bearing pressure-resistant memberas claimed in claim 1, wherein said bearing pressure-resistant member ismade of a mechanical structural steel which contains Cr in an amountranging from 1.2 to 3.2 mass percent and Mo in an amount ranging from0.25 to 2.0 mass percent, and treated by either of carburizing andcarbonitriding, said carbide comprising M₂₃C₆ carbide containing atleast Cr.
 5. The bearing pressure-resistant member as claimed in claim1, wherein an amount of C contained in the bearing pressure-resistantmember is enhanced to said C content by either of carburizing andcarbonitriding.
 6. The bearing pressure-resistant member as claimed inclaim 1, wherein an amount of C contained in the bearingpressure-resistant member is enhanced to said C content by either of gascarburizing and gas carbonitriding, and a whole amount of H₂ releasedover a temperature range of 100° C. to 900° C. is limited to not morethan 0.2 ppm.
 7. The bearing pressure-resistant member as claimed inclaim 6, wherein said bearing pressure-resistant member is made of amechanical structural steel containing Cr in an amount ranging from 1.2to 3.2 mass percent and Mo in an amount ranging from 0.25 to 2.0 masspercent, said carbide comprising M₂₃C₆ carbide containing at least Cr.8. A process for making a bearing pressure-resistant member, comprising:subjecting a workpiece containing C to either of gas carburizing and gascarbonitriding to enhance a C content in an outer surface of theworkpiece to 0.6 to 1.5 mass percent; holding the workpiece at a firsttemperature not more than an Ac₁ transformation point under reducedpressure; heating the workpiece to a second temperature not less thanthe Ac₁ transformation point under reduced pressure, followed by holdingthe workpiece at the second temperature; and subjecting the workpiece toquenching.
 9. The process as claimed in claim 8, wherein the workpieceis made of a mechanical structural steel containing Cr in an amountranging from 1.2 to 3.2 mass percent and Mo in an amount ranging from0.25 to 2.0 mass percent, the first temperature is in a range of 600° C.to 750° C. which is reached by heating the workpiece at a rate of 0.2°C. to 30° C. per minute in a temperature range of 500° C. to 650° C.,and the second temperature is in a range not less than the Ac₁transformation point but not more than a predetermined temperature Trepresented by the following formula: T(° C.)=675+120.Si(%)−27.Ni(%)+30.Cr(%)+215.Mo(%)−400V(%).
 10. The process as claimed in claim 9,wherein the rate of heating the workpiece in the temperature range of500° C. to 650° C. is in a range of 0.2° C. to 5° C. per minute.
 11. Aprocess for making a bearing pressure-resistant member, comprising:heating a workpiece made of a mechanical structural steel containing Cin an amount ranging from 0.6 to 1.5 mass percent, to a firsttemperature of 600° C. to 750° C. wherein the workpiece is heated at arate of 0.2° C. to 30° C. per minute in a temperature range of 500° C.to 650° C.; holding the workpiece at the first temperature; heating theworkpiece to a second temperature not less than an Ac₁ transformationpoint and not more than an Acm transformation point, followed by holdingthe workpiece at the second temperature; and subjecting the workpiece toquenching.
 12. The process as claimed in claim 11, further comprisingsubjecting the workpiece to either of carburizing and carbonitriding toenhance the C content in an outer surface of the workpiece to 0.6 to 1.5mass percent before heating the workpiece to the first temperature. 13.The process as claimed in claim 11, wherein the rate of heating theworkpiece in the temperature range of 500° C. to 650° C. is in a rangeof 0.2° C. to 5° C. per minute.
 14. A process for making a bearingpressure-resistant member, comprising: heating a workpiece made of amechanical structural steel containing C in an amount ranging from 0.6to 1.5 mass percent, Cr in an amount ranging from 1.2 to 3.2 masspercent, and Mo in an amount ranging from 0.25 to 2.0 mass percent, to afirst temperature of 600° C. to 750° C. under reduced pressure whereinthe workpiece is heated at a rate of 0.2° C. to 30° C. per minute in atemperature range of 500° C. to 650° C. under reduced pressure; holdingthe workpiece at the first temperature; heating the workpiece to asecond temperature not less than an Ac₁ transformation point and notmore than a predetermined temperature T represented by the followingformula: T(° C.)=675+120.Si(%)−27.Ni(%)+30.Cr(%)+215.Mo(%)−400V(%);holding the workpiece at the second temperature; and subjecting theworkpiece to quenching.
 15. The process as claimed in claim 14, furthercomprising subjecting the workpiece to either of carburizing andcarbonitriding to enhance the C content in an outer surface of theworkpiece to 0.6 to 1.5 mass percent before heating the workpiece to thefirst temperature.
 16. The process as claimed in claim 14, wherein therate of heating the workpiece in the temperature range of 500° C. to650° C. is in a range of 0.2° C. to 5° C. per minute.